In guns, such as shotguns, rifles, pistols or the like, it is important to have the proper space between the breech face and the cartridge head. This space is referred to in the gunsmithing business as the "head space." It is the distance, in thousandths of an inch between the cartridge head and the breech face or bolt face. Since there is no convenient way to measure the head space directly, it has become a common practice to calculate it from two other dimensions.
The barrel of a gun is typically connected to the receiver of a gun by a threaded arrangement wherein the threaded male portion on the barrel is threadably received into a female threaded portion of the receiver. In this arrangement, a planar portion of the barrel abuts a planar portion of the receiver when the barrel is completely threaded into the receiver. By measuring the distance (y) between this planar abutting surface of the receiver from the breech face, such as by use of a depth micrometer, and then by making another measurement of the distance between the other abutting planar surface on the barrel and the distance that a shell or cartridge would extend out from the barrel when inserted into the chamber, the head space can be quickly calculated by subtracting these two distances. In practice, rather than measuring the distance between the planar surface on the barrel and the outwardmost extension of a live round placed in the firing chamber, a head space gauge, which has the shape of a shell or cartridge in the critical dimension to be measured, would be placed into the chamber and the distance (x) between the outwardmost planar surface thereon and the planar surface of the barrel would be measured, also with a depth micrometer, for example.
One of the problems, however, with using a depth micrometer is that it is difficult to hold the depth micrometer so that the axis of the measuring post extending therefrom is always kept completely parallel to the longitudinal axis of the head space gauge. If there is a substantial deviation from this desired parallel relationship during the measurement of dimension (x) referred to above, then the calculation of the head space will be wrong by the amount of this error.
Consequently, there is a need for a device which will more dependably measure the distance (x) referred to above, reducing the danger that it will be measured incorrectly.